Focused ion beam (FIB) is widely used as a material removal tool for applications ranging from electron microscope sample preparation to nanopore processing for DNA sequencing. Despite the wide spread use of FIB, the basic material removal mechanisms are not well understood and may depend upon FIB operations. We present the first complete atomistic simulation of high-flux FIB using large-scale parallel molecular dynamics (MD) simulations of nanopore fabrication in freestanding thin films. We focus on the root mechanisms as described by large-scale MD simulations of FIB and describe the role of explosive boiling and Marangoni effect as a material removal and rearrangement mechanism and the mixing and transport that occur at the atomic scale.
Nanopore fabrication using FIB is typically understood to occur via sputter erosion. While this theory may describe low-flux systems, where individual ion impacts are sufficiently separated in time to consider them as independent events, it cannot explain the thermal events observed during high flux simulations. A dimensionless number is introduced, which is constructed from the key variables including material properties and FIB parameters. This number suggests strong thermal effects when it is greater than unity. Similarly, our detailed MD simulations suggest that for ion beam fluxes above a threshold level, the dominant mechanism of material removal changes to a significantly accelerated, thermally dominated process, consistent with our dimensional analysis. During this time, the target is heated faster than it cools, leading to melting, with local temperatures approaching the critical temperature. This leads to an explosive boiling of the target material with spontaneous bubble formation. Atomic mass is rapidly rearranged via bubble growth and coalescence and material removal is orders of magnitude faster than would occur by simple sputtering.
For a range of ion intensities in a realistic configuration, a recirculating melt region develops, which is seen to flow at high speed though symmetrically rather than driven by the ion momentum flux. Relevant length and time scales and estimated physical properties of silicon under these extreme conditions suggest that thermocapillary effects are important. A flow model with a Marangoni forcing term, based upon the temperature gradient from the atomistic simulation, confirms the presence of thermocapillary effect by reproducing the flow.